One of my favorite game mechanics in any game with RPG elements most definitely has to be status effects. I'm not sure of what form in which these exist in Demigod (though, I assume they DO exist in some form by the While-Immolated skills from the Torchbearer), but I would love to see an Assassin (and maybe some recruitable from Generals) that makes use of several Status Effects.
Burn damage over time, massive slowdown of advancing units over an area, blindness to reduce accuracy of archers or confuse melee units caused by the focused ray of light from the Rook...sure, they all sound great...
Hireable mages of some sort with the ability to cause units to fall asleep, front-line berserkers with the ability to stun groups with a massive swing, archers with alchemic arrows that release clouds of poisons, or harpies with a shrill scream that deafens the General's units, preventing them from taking orders for a time.
Beyond negative status effects, positive effects are also a great addition to games with RPG elements, and buffers and support skills (Green Mages, as Final Fantasy calls them now) are always a welcome addition. Barbaric warcries, holy auras, or mystic enchantments -- whatever it takes to raise stats or morale of your forces to give them the much-needed edge in battle -- are generally great bolsters to your side in a fight...
But what if there were an Empathic Assassin lurking nearby? You unleash all your boosts to your forces...only to see them take effect on an enemy Assassin as well -- as a result of his passive ability -- Clairsentience. Not only are you helping yourself, you've become a Benedict Arnold through no fault of your own, setting up the enemy Assassin to rip through your newly-boosted forces.
So you'll try to fight him (and the cavalry that rode in with him) off with some of the aforementioned negative status effects, eh? Well, you may wanna watch out for Empathic Contagion. Status effects of teammates' nearby units get completely absorbed by the Assassin, taking a negative toll on him alone, then projected onto his nearby foes.
If your Assassin is affected, and you think you want to chug that Elixir that cures all status effects, you better hope your newly discovered enemy isn't using the skill Clairgustance, where any expendable potion used within a certain radius of the Assassin also takes effect on him...
Beware the Echidna, lurking in wait to steal your spirit and soul.